In recent years, GaN-based compound semiconductor materials have drawn attention as semiconductor materials for light-emitting devices which emit short-wavelength light. A GaN-based compound semiconductor is formed on any of a variety of oxide substrates such as a sapphire single crystal or a III-V Group compound substrate, through a metal organic chemical vapor deposition method (MOCVD method), a molecular beam epitaxy method (MBE method), or a similar method.
One characteristic of the GaN-based compound semiconductor material is that the current diffusing in the lateral direction is small. Although the phenomenon has not been completely elucidated, one conceivable reason is that the current diffusion is affected by a large number of dislocations propagating through the epitaxial crystal layers in a direction from a substrate to the top surface. In addition, as the electric resistance of a p-type GaN-based compound semiconductor is very high as compared with that of an n-type GaN-based compound semiconductor, when a metal is merely laminated on the p-type GaN-based compound semiconductor, a current is scarcely diffused in the lateral direction in the p-type GaN-based compound semiconductor. Therefore, when an LED structure having a p-n junction is formed of the GaN-based compound semiconductor, the light emission is limited to a region directly below the electrodes.
To solve this problem, electron beam irradiation or high-temperature annealing is applied to decrease the resistivity of p-type semiconductor layer and thereby intensify the diffusibility of current. However, the electron beam irradiation involves an unduly high production cost because the apparatus is very expensive. Also, uniform processing in the wafer plane is difficult to attain. In high-temperature annealing, a process at 900° C. or more is necessary for markedly bringing out the effect but, in this process, the crystal structure of GaN starts to decompose and the voltage property in the reverse direction may deteriorate due to desorption of nitrogen.
There has been also proposed a technique where Ni and Au, each on the order of tens of nm, are stacked as positive electrodes on the p-type semiconductor layer and alloyed in an oxygen atmosphere to accelerate the reduction of resistance of the p-type semiconductor layer and form a positive electrode having transparency and ohmic property (see, Japanese Patent No. 2,803,742).
However, alloying in an oxygen atmosphere brings about formation of an oxide layer on the exposed surface of n-type GaN layer and this layer affects the ohmic property of the negative electrode. Also, the Au/Ni electrode resulting from alloying in an oxygen atmosphere has a network structure and therefore, uneven light emission readily occurs or the mechanical strength is too low and the electrode requires covering with a protective layer, giving rise to high production cost. Furthermore, since Ni is heat-treated in an oxygen atmosphere, an oxide of Ni covers the surface and when a pad electrode is formed on the transparent electrode, the adhesive property thereof is poor and a sufficiently high bonding strength cannot be obtained.
There has been also proposed a technique where Pt is formed as the positive electrode and heat-treated in an oxygen-containing atmosphere to effect the reduction of resistance of a p-type semiconductor layer and the alloying at the same time (see, Japanese Unexamined Patent Publication (Kokai) No. 11-186605). However, this method also has the above-described problems because the heat treatment is performed in an oxygen atmosphere. Furthermore, the thickness of the Pt layer must be very small (5 nm or less) so as to provide a transparent electrode of Pt alone and, in turn, the Pt layer comes to have a high electric resistance. As a result, even when the resistance of the Pt layer is decreased by heat treatment, the current diffuses poorly and non-uniform light emission occurs, giving rise to elevation of forward voltage (VF) and reduction of emission intensity.